High-density molding device and high-density molding method for mixed powder

ABSTRACT

A first die is filled with a mixed powder including a lubricant. A first pressure is applied to the mixed powder to form a mixed powder intermediate compressed body having a density ratio of 85 to 96%, provided that the maximum density of the mixed powder intermediate compressed body that can be molded by applying the first pressure is 100%. The mixed powder intermediate compressed body is heated to the melting point of the lubricant powder. The mixed powder intermediate compressed body is placed in a second die that is pre-heated to the melting point. A second pressure is applied to the mixed powder intermediate compressed body in the second die to form a high-density mixed powder final compressed body.

TECHNICAL FIELD

The present invention relates to a high-density molding method and ahigh-density molding system that can form a green compact having highdensity (e.g., 7.75 g/cm³) by pressing a mixed powder twice.

BACKGROUND ART

Powder metallurgy is a technique that normally presses (compresses) ametal powder to form a green compact having a given shape, and heats thegreen compact to a temperature around the melting point of the metalpowder to promote intergranular coupling (solidification) (i.e.,sintering process). This makes it possible to inexpensively produce amechanical part that has a complex shape and high dimensional accuracy.

An improvement in mechanical strength of a green compact has beendesired in order to deal with a demand for a further reduction in sizeand weight of mechanical parts. When a green compact is subjected to ahigh temperature, the magnetic properties of the green compact maydeteriorate. Therefore, the subsequent high-temperature treatment(sintering process) may be omitted when producing a magnetic-core greencompact, for example. In other words, a method that improves mechanicalstrength without performing a high-temperature treatment (sinteringprocess) has been desired.

The mechanical strength of a green compact increases significantly(hyperbolically) as the density of the green compact increases. Forexample, a method that mixes a lubricant into a metal powder, andpress-molds the metal powder while achieving a reduction in frictionresistance has been proposed as a typical high-density molding method(e.g., JP-A-1-219101 (Patent Literature 1)). A mixed powder prepared bymixing a lubricant with a basic metal powder in a ratio of about 1 wt %is normally press-molded. Various other methods have been proposed toachieve higher density. These methods can be roughly classified into amethod that improves the lubricant and a method that improves the pressmolding/sintering process.

Examples of the method that improves the lubricant include a method thatutilizes a composite of carbon molecules obtained by combining aball-like carbon molecule with a sheet-like carbon molecule as thelubricant (e.g., JP-A-2009-280908 (Patent Literature 2)), and a methodthat utilizes a lubricant having a penetration at 25° C. of 0.3 to 10 mm(e.g., JP-A-2010-37632 (Patent Literature 3)). These methods aim atreducing the friction resistance between the metal powder particles andthe friction resistance between the metal powder and a die.

Examples of the method that improves the press molding/sintering processinclude a warm molding/sinter powder metallurgical technique (e.g.,JP-A-2-156002 (Patent Literature 4)), a facilitated handling warmmolding powder metallurgical technique (e.g., JP-A-2000-87104 (PatentLiterature 5)), a double press/double sinter powder metallurgicaltechnique (e.g., JP-A-4-231404 (Patent Literature 6)), and a singlepress/sinter powder metallurgical technique (e.g., JP-A-2001-181701(Patent Literature 7)).

According to the warm molding/sinter powder metallurgical technique, ametal powder into which a solid lubricant and a liquid lubricant aremixed is pre-heated to melt part or the entirety of the lubricant, anddisperse the lubricant between the metal powder particles. Thistechnique thus reduces the inter-particle friction resistance and thefriction resistance between the particles and a die to improveformability. According to the facilitated handling warm molding powdermetallurgical technique, a mixed powder is pressed before performing awarm molding step to form a primary molded body having low density(e.g., density ratio: less than 76%) that allows handling (primarymolding step), and the primary formed body is subjected to a secondarymolding step at a temperature lower than the temperature at which blueshortness occurs while breaking the primary molded body to obtain asecondary molded body (green compact). According to the doublepress/double sinter powder metallurgical technique, an iron powdermixture that contains an alloying component is compressed in a die toobtain a compressed body, the compressed body (green compact) ispresintered at 870° C. for 5 minutes, and compressed to obtain apresintered body, and the presintered body is sintered at 1000° C. for 5minutes to obtain a sintered body (part). According to the singlepress/sinter powder metallurgical technique, a die is pre-heated, and alubricant is caused to electrically adhere to the inner side of the die.The die is filled with a heated iron-based powder mixture (iron-basedpowder+lubricant powder), and the powder mixture is press-molded at agiven temperature to obtain an iron-based powder molded body. Theiron-based powder molded body is sintered, and subjected to brightquenching and annealing to obtain an iron-based sintered body.

The density of the green compact achieved by the methods that improvethe lubricant and the methods that improve the press molding/sinteringprocess is about 7.4 g/cm³ (94% of the true density) at a maximum. Thegreen compact exhibits insufficient mechanical strength when the densityof the green compact is 7.4 g/cm³ or less. Since oxidation proceedscorresponding to the temperature and the time when applying thesintering process (high-temperature atmosphere), the lubricant coatedwith the powder particles burns, and a residue occurs, whereby thequality of the green compact obtained by press molding deteriorates.Therefore, it is considered that the density of the green compact is 7.3g/cm³ or less. The methods that improve the lubricant and the methodsthat improve the press molding/sintering process are complex, mayincrease cost, and have a problem in that handling of the material isdifficult or troublesome (i.e., it may be impractical).

In particular, when producing a magnetic core for an electromechanicaldevice (e.g., motor or transformer) using a green compact, asatisfactory magnetic core may not be produced when the density of thegreen compact is 7.3 g/cm³ or less. It is necessary to further increasethe density of a green compact in order to reduce loss (iron loss andhysteresis loss), and increase magnetic flux density (see the documentpresented by Toyota Central R & D Labs., Inc. in Autumn Meeting of JapanSociety of Powder and Powder Metallurgy, 2009). Even when the density ofthe magnetic core is 7.5 g/cm³, for example, the magnetic properties andthe mechanical strength of the magnetic core may be insufficient inpractice.

A double molding/single sinter (anneal) powder metallurgical technique(e.g., JP-A-2002-343657 (Patent Literature 8)) has been proposed as amethod for producing a magnetic-core green compact. This powdermetallurgical technique is based on the fact that a magnetic metalpowder that is coated with a coating that contains a silicone resin anda pigment does not show a decrease in insulating properties even if themagnetic metal powder is subjected to a high-temperature treatment.Specifically, a dust core is produced by pre-molding a magnetic metalpowder that is coated with a coating that contains a silicone resin anda pigment to obtain a pre-molded body, subjecting the pre-molded body toa heat treatment at 500° C. or more to obtain a heat-treated body, andcompression-molding the heat-treated body. If the heat treatmenttemperature is less than 500° C., breakage may occur during compressionmolding. If the heat treatment temperature is more than 1000° C., theinsulating coating may be decomposed (i.e., the insulating propertiesmay be impaired). Therefore, the heat treatment temperature is set to500 to 1000° C. The high-temperature treatment is performed undervacuum, an inert gas atmosphere, or a reducing gas atmosphere in orderto prevent oxidation of the pre-molded body. A dust core having a truedensity of 98% (7.7 g/cm³) may be produced as described above.

SUMMARY OF THE INVENTION Technical Problem

However, the double molding/single sinter powder metallurgical technique(Patent Literature 8) is very complex, individualized, and difficult toimplement as compared with the other techniques, and significantlyincreases the production cost. The double molding/single sinter powdermetallurgical technique subjects the pre-molded body to a heat treatmentat 500° C. or more. The heat treatment is performed under such anatmosphere in order to prevent a situation in which the quality of thedust core deteriorates. Therefore, the double molding/single sinterpowder metallurgical technique is not suitable for mass production. Inparticular, when using a vitreous film-coated magnetic metal powder, thevitreous material may be modified/melted.

The above methods and systems (Patent Literatures 1 to 8) can implementa sintering process at a relatively high temperature. However, thedetails of the press molding step achieved using the above methods andsystems are unclear. Moreover, attempts to achieve a further improvementin connection with the specification and the functions of the pressmolding device, the relationship between pressure and density, and ananalysis of the limitations thereof, have not been made.

As described above, a further increase in mechanical strength has beendesired along with a reduction in size and weight of mechanical partsand the like, and there is an urgent need to develop a method and asystem that can reliably, stably, and inexpensively produce ahigh-density green compact (particularly a magnetic-core high-densitygreen compact).

An object of the invention is to provide a mixed powder high-densitymolding method and a mixed powder high-density molding system that canproduce a high-density green compact while significantly reducing theproduction cost by press-molding a mixed powder twice with a heatingstep interposed therebetween.

Solution to Problem

A green compact has been normally produced by a powder metallurgicaltechnique, and subjected to a sintering process performed at a hightemperature (e.g., 800° C. or more). However, such a high-temperaturesintering process consumes a large amount of energy (i.e., increasescost), and is not desirable from the viewpoint of environmentalprotection.

The press molding process molds a mixed powder to have a specific shape,and has been considered to be a mechanical process that is performed inthe preceding stage of the high-temperature sintering process. Thehigh-temperature sintering process is exceptionally omitted whenproducing a magnetic-core green compact used for an electromagneticdevice (e.g., motor or transformer). This aims at preventing adeterioration in magnetic properties that may occur when the greencompact is subjected to a high-temperature process. Specifically, theresulting product inevitably has unsatisfactory mechanical strength.Since the density of the product is insufficient when mechanicalstrength is insufficient, the product also has insufficient magneticproperties.

It is possible to significantly promote industrial utilization andwidespread use of a green compact if a high-density green compact can beformed only by the press molding process without performing thehigh-temperature sintering process. The invention was conceived based onstudies of the effectiveness of a lubricant during pressing, thecompression limit when using a lubricant powder, the spatialdistribution of a lubricant powder in a mixed powder, the spatialdistribution of a basic metal powder and a lubricant powder, thebehavior of a basic metal powder and a lubricant powder, and the finaldisposition state of a lubricant, so that the actual production cyclemay be reduced while ensuring the safety of the device. The inventionwas also conceived based on analysis of the characteristics (e.g.,compression limit) of a normal press molding device, and the effects ofthe density of a green compact on strength and magnetic properties.

Specifically, the invention may provide a method that fills a first diewith a mixed powder prepared by mixing a lubricant powder into a basicmetal powder, molds an intermediate green compact having a true densityratio of 85 to 96% by performing a first press molding step whilemaintaining the lubricant in a powdery state, liquefies the lubricant byheating to change the state of the lubricant within the intermediategreen compact, and molds a high-density final green compact having adensity close to the true density by performing a second press moldingstep. In other words, the invention may provide a novel powdermetallurgical technique (i.e., a powder metallurgical technique thatperforms two press molding steps with a lubricant liquefaction stepinterposed therebetween) that differs from a known powder metallurgicaltechnique that necessarily requires a high-temperature sinteringprocess, and may provide an epoch-making and practical method and systemthat can reliably and stably produce a high-density green compact at lowcost.

(1) According to a first aspect of the invention, a mixed powderhigh-density molding method includes:

filling a first die with a mixed powder prepared by mixing alow-melting-point lubricant powder into a basic metal powder;

applying a first pressure to the mixed powder in the first die to form amixed powder intermediate compressed body having a density ratio of 85to 96%, provided that a maximum density of the mixed powder intermediatecompressed body that can be molded by applying the first pressure is100%;

heating the mixed powder intermediate compressed body removed from thefirst die to the melting point of the lubricant powder;

placing the heated mixed powder intermediate compressed body in a seconddie; and

applying a second pressure to the mixed powder intermediate compressedbody in the second die to form a high-density mixed powder finalcompressed body.

(2) In the mixed powder high-density molding method as defined in (1),the lubricant powder may have a low melting point within the range of 90to 190° C.

(3) In the mixed powder high-density molding method as defined in (1) or(2), the second die may be pre-heated to the melting point before themixed powder intermediate compressed body is placed in the second die.

(4) In the mixed powder high-density molding method as defined in (1) or(2), the second pressure may be selected to be equal to the firstpressure.

(5) According to a second aspect of the invention, a mixed powderhigh-density molding system includes:

a mixed powder feeding device that can externally feed a mixed powderprepared by mixing a low-melting-point lubricant powder into a basicmetal powder;

a first press molding device that applies a first pressure to the mixedpowder, with which a first die has been filled using the mixed powderfeeding device, to form a mixed powder intermediate compressed body;

a heating device that heats the mixed powder intermediate compressedbody removed from the first die to the melting point of the lubricantpowder; and

a second press molding device that includes a second die that can bepre-heated to the melting point, and applies a second pressure to themixed powder intermediate compressed body that is placed in thepre-heated second die to form a high-density mixed powder finalcompressed body,

the first press molding device applying the first pressure to the mixedpowder in the first die to form the mixed powder intermediate compressedbody having a density ratio of 85 to 96%, provided that a maximumdensity of the mixed powder intermediate compressed body that can bemolded by applying the first pressure is 100%.

(6) In the mixed powder high-density molding system as defined in (5),the heating device and the second press molding device may be formed bya heating/press molding device that functions as the heating device andthe second press molding device, the heating/press molding device mayinclude a plurality of heating/press molding sub-devices, and each ofthe plurality of heating/press molding sub-devices may be selectivelyand sequentially operated in each cycle.

(7) The mixed powder high-density molding system as defined in (5) mayfurther include a pre-heating device that pre-heats the second die.

(8) The mixed powder high-density molding system as defined in (5) mayfurther include a workpiece transfer device that transfers the mixedpowder intermediate compressed body formed by the first press moldingdevice to the heating device, transfers the mixed powder intermediatecompressed body heated by the heating device to the second press moldingdevice, and transfers the mixed powder final compressed body formed bythe second press molding device to a discharge section.

Advantageous Effects of the Invention

The mixed powder high-density molding method as defined in (1) canreliably and stably produce a high-density green compact whilesignificantly reducing the production cost. It is also possible toreduce the actual production cycle while ensuring the safety of thedevice.

The mixed powder high-density molding method as defined in (2) makes itpossible to ensure that the lubricant produces a sufficient lubricatingeffect during the first press molding step while suppressing oxidationof the lubricant. It is also possible to selectively use a wide varietyof lubricants.

The mixed powder high-density molding method as defined in (3) makes itpossible to further improve the fluidity of the melted lubricant in alldirections during the second press molding step, and significantlyreduce the friction resistance between the basic metal particles and thefriction resistance between the particles and the second die.

The mixed powder high-density molding method as defined in (4) makes itpossible to easily implement the press molding step, facilitatehandling, and indirectly reduce the green compact production cost.

The mixed powder high-density molding system as defined in (5) canreliably implement the mixed powder high-density molding method asdefined in any one of (1) to (4), can be easily implemented, andfacilitates handling.

The mixed powder high-density molding system as defined in (6) makes itpossible to simplify the system as compared with the configuration asdefined in (5). It is also possible to simplify the production line, andfurther facilitate handling.

The mixed powder high-density molding system as defined in (7) makes itpossible to allow the temperature of the mixed powder intermediatecompressed body to be within a given temperature range even when thetemperature of the mixed powder intermediate compressed body decreasesuntil the final green compact molding start timing occurs, and achieve agood molding effect.

Since the mixed powder high-density molding system as defined in (8)includes the workpiece transfer device, it is possible to reliablytransfer the workpiece in the area between the first press moldingdevice and the heating device, the area between the heating device andthe second press molding device, and the area between the second pressmolding device and the discharge section.

Further features and advantageous effects of the invention will becomeapparent from the following description.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a high-density molding method accordingto one embodiment of the invention.

FIG. 2 is a front view illustrating a high-density molding systemaccording to a first embodiment of the invention (and its operation).

FIG. 3A is a view illustrating a mixed powder high-density moldingoperation according to the first embodiment of the invention, andillustrates a state in which an intermediate green compact is moldedusing a first die.

FIG. 3B is a view illustrating a mixed powder high-density moldingoperation according to the first embodiment of the invention, andillustrates a state in which a first die is filled with a mixed powder.

FIG. 4 is a graph illustrating the relationship between pressure anddensity obtained at the pressure (first embodiment of the invention),wherein characteristics A (curve) indicate a molding state using a firstdie, and characteristics B (straight line) indicate a molding stateusing a second die.

FIG. 5A is an external perspective view illustrating a final greencompact (intermediate green compact) according to the first embodimentof the invention having a ring-like shape.

FIG. 5B is an external perspective view illustrating a final greencompact (intermediate green compact) according to the first embodimentof the invention having a cylindrical shape.

FIG. 5C is an external perspective view illustrating a final greencompact (intermediate green compact) according to the first embodimentof the invention having a narrow round shaft shape.

FIG. 5D is an external perspective view illustrating a final greencompact (intermediate green compact) according to the first embodimentof the invention having a disc-like shape.

FIG. 5E is an external perspective view illustrating a final greencompact (intermediate green compact) according to the first embodimentof the invention having a complex shape.

FIG. 6 is a front view illustrating a high-density molding systemaccording to a second embodiment of the invention, and its operation.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the invention are described in detail belowwith reference to the drawings.

First Embodiment

As illustrated in FIGS. 1 to 5E, a mixed powder high-density moldingsystem 1 includes a mixed powder feeding device 10, a first pressmolding device 20, a heating device 30, and a second press moldingdevice 40, and stably and reliably implements a mixed powderhigh-density molding method that includes a mixed powder-filling step(PR1) that fills a first die (lower die 21) with a mixed powder 100 thatis prepared by mixing a low-melting-point lubricant powder with a basicmetal powder, an intermediate green compact-forming step (PR2) thatapplies a first pressure (P1) to the mixed powder in the first die(lower die 21) to form a mixed powder intermediate compressed body(hereinafter may be referred to as “intermediate green compact 110”)having a density ratio of 85 to 96%, provided that the maximum densityof the mixed powder intermediate compressed body that can be molded byapplying the first pressure P1 is 100%, a heating step (PR3) that heatsthe intermediate green compact 110 removed from the first die (lower die21) to the melting point of the lubricant powder, a step (PR4) thatplaces the heated intermediate green compact 110 in a second die (lowerdie 41), and a final green compact-forming step (PR5) that applies asecond pressure P2 to the intermediate green compact 110 in the seconddie (lower die 41) to form a high-density mixed powder final compressedbody (hereinafter may be referred to as “final green compact 120”).

The mixed powder 100 is a mixture of the basic metal powder and thelow-melting-point lubricant powder. The basic metal powder may includeonly one type of main metal powder, or may be a mixture of one type ofmain metal powder and one or more types of alloying component powder.The expression “low melting point” used herein in connection with thelubricant powder refers to a temperature (melting point) that issignificantly lower than the melting point (temperature) of the basicmetal powder, and can significantly suppress oxidation of the basicmetal powder.

As illustrated in FIG. 2 that shows the high-density molding system 1,the mixed powder feeding device 10 is disposed on the leftmost side(upstream side) of a high-density molding line. The mixed powder feedingdevice 10 feeds the mixed powder 100 to the first die (lower die 21)included in a first press molding device 20 to fill a cavity 22 of thefirst die (lower die 21) with the mixed powder 100. The mixed powderfeeding device 10 has a function of holding a constant amount of themixed powder 100, and a function of feeding a constant amount of themixed powder 100. The mixed powder feeding device 10 can selectivelymove between the initial position (i.e., the position indicated by thesolid line in FIGS. 2, 3A, and 3B) and the position over the first die(lower die 21) (i.e., the position indicated by the broken line in FIGS.3A and 3B).

Since it is important to uniformly and sufficiently fill the first die(lower die 21) with the mixed powder 100, the mixed powder 100 must bein a dry state. Specifically, since the shape of the internal space(cavity 22) of the first die (lower die 21) corresponds to the shape ofthe product, it is necessary to uniformly and sufficiently fill thefirst die with the mixed powder 100 in order to ensure the dimensionalaccuracy of the intermediate green compact 110, even if the product hasa complex shape, or has a narrow part.

The configuration (dimensions and shape) of the final green compact 120(intermediate green compact 110) is not particularly limited. FIGS. 5Ato 5E illustrate examples of the configuration (dimensions and shape) ofthe final green compact 120 (intermediate green compact 110). FIG. 5Aillustrates the final green compact 120 (intermediate green compact 110)having a ring-like shape, FIG. 5B illustrates the final green compact120 (intermediate green compact 110) having a cylindrical shape, FIG. 5Cillustrates the final green compact 120 (intermediate green compact 110)having a narrow round shaft shape, FIG. 5D illustrates the final greencompact 120 (intermediate green compact 110) having a disc-like shape,and FIG. 5E illustrates the final green compact 120 (intermediate greencompact 110) having a complex shape.

Specifically, an upper die (upper punch) 25 and the cavity 22 of thelower die 21 of the first press molding device 20 have a shapecorresponding to the configuration (shape) of the intermediate greencompact 110. When the intermediate green compact 110 has theconfiguration (shape) illustrated in FIG. 5A, 5B, 5C, 5D, or 5E, theupper die (upper punch) 25 and the cavity 22 of the lower die 21 have ashape corresponding to the configuration (shape) of the intermediategreen compact 110. When the intermediate green compact 110 has thering-like shape illustrated in FIG. 5A, the upper die (upper punch) 25has a cylindrical shape, and the lower die 21 has a hollow cylindricalshape (see FIGS. 2, 3A, and 3B). When the intermediate green compact 110has the cylindrical shape illustrated in FIG. 5B, the upper die (upperpunch) 25 has a solid cylindrical shape, and the lower die 21 has ahollow cylindrical shape. This also applies to the case where theintermediate green compact 110 has the narrow round shaft shapeillustrated in FIG. 5C, or the disc-like shape illustrated in FIG. 5D(except for the depth). When the intermediate green compact 110 has thecomplex shape illustrated in FIG. 5E, the upper die (upper punch) 25 andthe lower die 21 have the corresponding complex shape. This also appliesto an upper die (upper punch) 45 and a cavity 42 of a lower die 41 ofthe second press molding device 40.

A solid lubricant that is in a dry state (fine particulate) (i.e.,powdery state) at room temperature is used as the lubricant that is usedto reduce the friction resistance between the basic metal particles andthe friction resistance between the basic metal powder and the innerside of the die. For example, since the mixed powder 100 exhibits highviscosity and low fluidity when using a liquid lubricant, it isdifficult to uniformly and sufficiently fill the first die with themixed powder 100.

It is also necessary for the lubricant to be solid and stably maintain agiven lubricating effect during the intermediate green compact moldingstep that is performed using the first die (lower die 21) at roomtemperature while applying the first pressure P1. The lubricant muststably maintain a given lubricating effect even if the temperature hasincreased to some extent as a result of applying the first pressure P1.

On the other hand, the melting point of the lubricant powder must besignificantly lower than the melting point of the basic metal powderfrom the viewpoint of the relationship with the heating step (PR3)performed after the intermediate green compact molding step, andsuppression of oxidation of the basic metal powder.

In the first embodiment, the lubricant powder has a low melting pointwithin the range of 90 to 190° C. The lower-limit temperature (90° C.)is selected to be higher to some extent than the upper-limit temperature(80° C.) of a temperature range (e.g., 70 to 80° C.) that is not reachedeven if the temperature has increased to some extent during theintermediate green compact molding step, while taking account of themelting point (e.g., 110° C.) of other metallic soaps. This prevents asituation in which the lubricant powder is melted (liquefied) and flowsout during the intermediate green compact molding step.

The upper-limit temperature (e.g., 190° C.) is selected to be a minimumvalue from the viewpoint of lubricant powder selectivity, and isselected to be a maximum value from the viewpoint of suppression ofoxidation of the basic metal powder during the heating step.Specifically, it should be understood that the lower-limit temperatureand the upper-limit temperature of the above temperature range (90 to190° C.) are not threshold values, but are boundary values.

This makes it possible to selectively use an arbitrary metallic soap(e.g., zinc stearate or magnesium stearate) as the lubricant powder.Note that a viscous liquid such as zinc octylate cannot be used sincethe lubricant must be in a powdery state.

In the first embodiment, a zinc stearate powder having a melting pointof 120° C. is used as the lubricant powder. Note that the invention doesnot employ a configuration in which a lubricant having a melting pointlower than the die temperature during press molding is used, and thepress molding step is performed while melting (liquefying) the lubricant(see Patent Literature 7). If the lubricant is melted and flows outbefore completion of molding of the intermediate green compact 110,lubrication tends to be insufficient during the molding step, andsufficient press molding cannot be performed reliably and stably.

The lubricant powder is used in an amount that is selected based on anempirical rule determined by experiments and actual production. Thelubricant powder is used in an amount of 0.08 to 0.23 wt % based on thetotal amount of the mixed power taking account of the relationship withthe intermediate green compact-forming step (PR2). When the amount ofthe lubricant powder is 0.08 wt %, the lubricating effect can bemaintained when molding the intermediate green compact 110. When theamount of the lubricant powder is 0.23 wt %, the desired compressionratio can be obtained when forming the intermediate green compact 110from the mixed powder 100.

A practical amount of the lubricant powder must be determined takingaccount of the true density ratio of the intermediate green compact 110that is molded in the first die (lower die 21) while applying the firstpressure, and a sweating phenomenon that occurs in the second die (lowerdie 41). It is also necessary to prevent dripping (dripping phenomenon)of the liquefied lubricant from the die toward the outside that causes adeterioration in the work environment. In the first embodiment, sincethe true density ratio (i.e., the ratio with respect to the true density(=100%)) of the intermediate green compact 110 is set to 80 to 90%, theratio (amount) of the lubricant powder is set to 0.1 to 0.2 wt %. Theupper limit (0.2 wt %) is determined from the viewpoint of preventingthe dripping phenomenon, and the lower limit (0.1 wt %) is determinedfrom the viewpoint of ensuring a necessary and sufficient sweatingphenomenon. The ratio (amount) of the lubricant powder is very small ascompared with the related-art example (1 wt %), and the industrialapplicability can be significantly improved.

It is very important to prevent the dripping phenomenon during actualproduction. A large amount of lubricant powder tends to be mixed in theplanning stage and the research stage in order to prevent a situation inwhich the lubricant powder runs short from the viewpoint of reducingfrictional resistance during pressing. Since whether or not a highdensity of more than 7.3 g/cm³ can be achieved is determined by trialand error, for example, a situation in which excess lubricant isliquefied and flows out from the die is not taken into consideration.The dripping phenomenon is also not taken into consideration. Sincedripping of the liquefied lubricant increases the lubricant cost,decreases productivity due to a deterioration in the work environment,and increases the burden imposed on the workers, it is impossible toensure practical and widespread use without preventing the drippingphenomenon.

When the intermediate green compact 110 obtained by compressing themixed powder 100 including 0.2 wt % of the lubricant powder to have atrue density ratio of 80% is heated to the melting point of thelubricant powder in the heating step (PR3), the powder lubricantscattered in the intermediate green compact 110 is melted to fill thevoids between the metal powder particles, passes through the voidsbetween the metal powder particles, and uniformly exudes through thesurface of the intermediate green compact 110. Specifically, thesweating phenomenon occurs. When the intermediate green compact 110 iscompressed in the second die (lower die 41) by applying the secondpressure P2, the frictional resistance between the basic metal powderand the inner wall of the cavity is significantly reduced.

The sweating phenomenon similarly occurs when using the intermediategreen compact 110 obtained by compressing the mixed powder 100 including0.1 wt % of the lubricant powder to have a true density ratio of 90%, orwhen using the intermediate green compact 110 obtained by compressingthe mixed powder 100 including more than 0.1 wt % and less than 0.2 wt %of the lubricant powder to have a true density ratio of more than 80%and less than 90%. It is also possible to prevent the drippingphenomenon.

This makes it possible to produce a green compact (e.g., magnetic core)that can be molded to have high density, and has sufficient magneticproperties and mechanical strength, and prevent a situation in which thedie breaks. Moreover, the consumption of the lubricant can besignificantly reduced, and a situation in which the liquid lubricantdrips from the second die (lower die 41) can be prevented, so that agood work environment can be achieved. Since the green compactproduction cost can be reduced while improving productivity, theindustrial applicability can be significantly improved.

Note that Patent Literatures 1 to 8 are silent about the relationshipbetween the lubricant content and the compression ratio of the mixedpowder, and the dripping phenomenon and the sweating phenomenon that mayoccur depending on the lubricant content.

Patent Literature 5 (warm powder metallurgical technique) disclosesproducing a primary molded body having a density ratio of less than 76%in order to facilitate handling, and does not disclose technical groundsrelating to high-density molding and items that can be implemented.Since the secondary molded body is produced in Patent Literature 5 afterbreaking the primary molded body, Patent Literature 5 does not employ atechnical idea that achieves an increase in density through primarymolding and secondary molding.

The first press molding device 20 applies the first pressure P1 to themixed powder 100 with which the first die (lower die 21) has been filledusing the mixed powder feeding device 10, to form the mixed powderintermediate compressed body (intermediate green compact 110). In thefirst embodiment, the first press molding device 20 has a pressstructure.

As illustrated in FIG. 2, the first die device includes the lower die 21that is situated on the side of a bolster, and the upper die (punch) 25that is situated on the side of a slide 5. The cavity 22 of the lowerdie 21 has a shape (hollow cylindrical shape) corresponding to the shape(ring-like shape) of the intermediate green compact 110 illustrated inFIG. 5A. Specifically, the upper die (upper punch) 25 can be pushed intothe lower die 21 (cavity 22) (see FIGS. 2, 3A, and 3B (cylindricalshape)), and is moved upward and downward using the slide 5. A movablemember 23 is fitted into the lower side of the cavity 22 so that themovable member 23 can move in the vertical direction.

When the intermediate green compact 110 has the shape illustrated inFIG. 5B, 5C, 5D, or 5E, the upper die (upper punch) 25 and the cavity 22of the lower die 21 of the first press molding device 20 also have ashape corresponding to the shape of the intermediate green compact 110.This also applies to the upper die (upper punch) 45 and the cavity 42 ofthe lower die 41 of the second press molding device 40.

The movable member 23 is moved upward using a knockout pin (notillustrated in the drawings) that moves upward through a through-hole 24that is formed under a ground level GL. The intermediate green compact110 in the first die (lower die 21 (cavity 22)) can thus be moved upwardto a transfer level HL. The movable member 23 functions as a firstejection device for ejecting the intermediate green compact 110 in thefirst die (lower die 21) to the outside (transfer level HL). The movablemember 23 and the knockout pin are returned to the initial positionafter the intermediate green compact 110 has been transferred to theheating device 30. Note that the first ejection device may beimplemented using another device.

The relationship between the pressure P (first pressure P1) applied bythe first press molding device 20 and the density ratio (density ρ) ofthe resulting intermediate green compact 110 is described below withreference to FIG. 4. The horizontal axis indicates the pressure P usingan index. In the first embodiment, the maximum capacity (pressure P) is10 tons/cm² (horizontal axis index: 100). Reference sign Pb indicatesthe die breakage pressure at which the horizontal axis index is 140 (14tons/cm²). The vertical axis indicates the density ratio (density ρ)using an index. A vertical axis index of 100 corresponds to a truedensity ratio (density ρ) of 97% (7.6 g/cm³).

In the first embodiment, the basic metal powder is a magnetic-corevitreous insulating film-coated iron powder (true density: 7.8 g/cm³),the lubricant powder is a zinc stearate powder (0.1 to 0.2 wt %), andthe first pressure P1 is selected so that the mixed powder intermediatecompressed body can be compressed to have a true density ratio of 80 to90% corresponding to a vertical axis index of 82 to 92 (corresponding toa density ρ of 6.24 to 7.02 g/cm³).

A vertical axis index of 102 corresponds to a density ρ of 7.75 g/cm³and a true density ratio (density ρ) of 99%.

Note that the basic metal powder may be a magnetic-core iron-basedamorphous powder (magnetic-core Fe—Si alloy powder), a magnetic-coreiron-based amorphous powder, a magnetic-core Fe—Si alloy powder, a pureiron powder for producing mechanical parts, or the like.

The density ρ achieved by the first press molding device 20 increasesalong the characteristics A (curve) illustrated in FIG. 4 as the firstpressure P1 increases. The density ρ reaches 7.6 g/cm³ when thehorizontal axis index (first pressure is P1) is 100. The true densityratio is 97%. The density ρ increases to only a small extent even if thefirst pressure P1 is further increased. The die may break if the firstpressure P1 is further increased.

When the density ρ achieved by pressing at the maximum capacity of thepress molding device (press) is not satisfactory, it has been necessaryto provide a larger press. However, the density ρ increases to only asmall extent even if the maximum capacity is increased by a factor of1.5, for example. Therefore, it has been necessary to accept a lowdensity ρ (e.g., 7.5 g/cm³) when using an existing press.

It is possible to achieve a major breakthrough if the vertical axisindex can be increased from 100 (7.6 g/cm³) to 102 (7.75 g/cm³) bydirectly utilizing an existing press. Specifically, it is possible tosignificantly (hyperbolically) improve magnetic properties, and alsosignificantly improve mechanical strength if the density ρ can beincreased by 2%. Moreover, since a sintering process at a hightemperature can be made unnecessary, oxidation of the green compact canbe significantly suppressed (i.e., a decrease in magnetic coreperformance can be prevented).

In order to achieve the above breakthrough, the high-density moldingsystem 1 is configured so that the intermediate green compact 110 formedby the first press molding device 20 is heated to promote melting(liquefaction) of the lubricant, and the second press molding device 40then performs the second press molding process. A high density (7.75g/cm³) ρ that corresponds to a vertical axis index of 102 (see thecharacteristics B (straight line) illustrated in FIG. 4) can be achievedby pressing the intermediate green compact 110 using the second pressmolding device 40. The details thereof are described later in connectionwith the second press molding device 40.

The heating device 30 is a device that heats the mixed powderintermediate compressed body (intermediate green compact 110) removedfrom the first die (lower die 21) to the melting point of the lubricantpowder. As illustrated in FIG. 2, the heating device 30 includes a hotair generator (not illustrated in FIG. 2), a blow hood 31, anexhaust/circulation hood 33, and the like. The heating device 30 blowshot air against the intermediate green compact 110 that is positionedusing a wire-mesh holding member 32 to heat the intermediate greencompact 110 to the melting point (e.g., 120° C.) of the lubricantpowder.

The technical significance of the above low-temperature heat treatmentis described below in connection with the relationship with the firstpress molding process. The powder mixture 100 with which the lower die21 (cavity 22) is filled has an area in which the lubricant powder isrelatively thinly present (thin area), and an area in which thelubricant powder is relatively densely present (dense area) inconnection with the basic metal powder. The friction resistance betweenthe basic metal particles, and the friction resistance between the basicmetal powder and the inner side of the die can be reduced in the densearea. In contrast, the friction resistance between the basic metalparticles, and the friction resistance between the basic metal powderand the inner side of the die increase in the thin area.

When the first press molding device 20 applies a pressure to the mixedpowder, compressibility is predominant (i.e., compression easily occurs)in the dense area due to low friction. In contrast, compressibility ispoor (i.e., compression slowly occurs) in the thin area due to highfriction. Therefore, a compression difficulty phenomenon correspondingto the preset first pressure P1 occurs (i.e., compression limit). Inthis case, when the fracture surface of the intermediate green compact110 removed from the die 21 is magnified, the basic metal powder isintegrally pressure-welded in the dense area. However, the lubricantpowder is also present in the dense area. In the thin area, small spacesremain in the pressure-welded basic metal powder, and almost nolubricant powder is observed in the thin area.

Therefore, it is possible to form compressible spaces by removing thelubricant powder from the dense area, and improve the compressibility ofthe thin area by supplying the lubricant to the spaces formed in thethin area.

Specifically, the lubricant powder is melted (liquefied), and increasedin fluidity by heating the intermediate green compact 110 subjected tothe first press molding process to the melting point (e.g., 120° C.) ofthe lubricant powder. The lubricant that flows out from the dense areapenetrates through the peripheral area, and is supplied to the thinarea. This makes it possible to reduce the friction resistance betweenthe basic metal particles, and compress the spaces that have beenoccupied by the lubricant powder. It is also possible to reduce thefriction resistance between the basic metal powder and the inner side ofthe die.

The second press molding device 40 is a device that applies the secondpressure P2 to the heated intermediate green compact 110 placed in thesecond die (lower die 41) to form the high-density final green compact120.

In the first embodiment, a function of pre-heating the second die (lowerdie 41) is provided. Note that the high-density molding method accordingto the invention can be implemented without pre-heating the second dieas long as the temperature of the heated intermediate green compact 110is within a given temperature range in which no problem occurs until thefinal green compact molding start timing at which the second pressure P2is applied.

However, when the heat capacity of the intermediate green compact 110 issmall, or when it takes time to transfer the intermediate green compact110 to the second die, or the intermediate green compact 110 istransferred to the second die (lower die 41) along a long transfer path,or when the temperature of the heated intermediate green compact 110decreases until the final green compact molding start timing occurs dueto the composition of the mixed powder, the configuration (shape) of theintermediate green compact 110, or the like, a good molding effect canbe obtained by pre-heating the second die (lower die 41). A secondpre-heating device 47 (described later) is provided to pre-heat thesecond die (lower die 41).

In the first embodiment, the maximum capacity (pressure P) of the secondpress molding device 40 is the same as that (10 tons/cm²) of the firstpress molding device 20. The first press molding device 20 and thesecond press molding device 40 are configured as a single press, and theupper die 25 and the upper die 45 are moved upward and downward insynchronization using the common slide 5 illustrated in FIG. 2. Theabove configuration is economical, and can reduce the production cost ofthe final green compact 120.

As illustrated in FIG. 2, the second die device includes the lower die41 that is situated on the side of the bolster, and the upper die(punch) 45 that is situated on the side of the slide 5. The lower partof the cavity 42 of the lower die 41 has a shape (cylindrical shape)corresponding to the shape (ring-like shape) of the final green compact120, and the upper part of the cavity 42 has a slightly larger shape sothat the intermediate green compact 110 can be placed therein. The upperdie 45 can be pushed into the lower die 41 (cavity 42), and is movedupward and downward using the slide 5. A movable member 43 is fittedinto the lower side of the cavity 42 so that the movable member 43 canmove in the vertical direction. Note that the second die (lower die 41)and the first die (lower die 21) are adjusted in height (position)corresponding to the vertical difference in dimensions between thecompression targets (intermediate green compact 110 and final greencompact 120).

The movable member 43 is moved upward using a knockout pin (notillustrated in the drawings) that moves upward through a through-hole 44that is formed under the ground level GL. The final green compact 120 inthe second die (lower die 41 (cavity 42)) can thus be moved upward tothe transfer level HL. The movable member 43 functions as a secondejection device for ejecting the final green compact 120 in the seconddie (lower die 41 (cavity 42)) to the outside (transfer level HL). Notethat the second ejection device may be implemented using another device.The movable member 43 and the knockout pin are returned to the initialposition after the final green compact 120 has been discharged to adischarge chute 59, and a new intermediate green compact 110 has beenreceived from the heating device 30.

The second die (lower die 41) is provided with the per-heating device 47that can be changed in heating temperature. The pre-heating device 47heats (pre-heats) the second die (lower die 41 (cavity 42)) to themelting point (e.g., 120° C.) of the lubricant powder (zinc stearate)before the intermediate green compact 110 is placed in the second die(lower die 41 (cavity 42)). Therefore, the heated intermediate greencompact 110 can be placed in the second die (lower die 41 (cavity 42))without allowing the intermediate green compact 110 to cool. This makesit possible to ensure a lubricating effect while preventing a situationin which the lubricant that has been melted (liquefied) is solidified.In the first embodiment, the second pre-heating device 47 is implementedusing an electric heating system (heater). Note that the secondpre-heating device 47 may also be implemented using a hot oil/hot watercirculation heating device or the like.

The second pre-heating device 47 can heat the second die until the finalgreen compact 120 is obtained. Therefore, the fluidity of the meltedlubricant in all directions can be further improved during pressmolding, and the friction resistance between the basic metal particlesand the friction resistance between the basic metal particles and thesecond die (lower die 41 (cavity 42)) can be significantly reduced.

In the first embodiment, a pre-heating function (not illustrated in thedrawings) for pre-heating the first die (lower die 21) is also provided.Note that the high-density molding method according to the invention canbe implemented without pre-heating the intermediate green compact 110 bypre-heating the first die (lower die 21) before the heating step.

However, when the composition of the mixed powder 100 or theconfiguration (shape) of the intermediate green compact 110 is unique,or when the heat capacity of the intermediate green compact 110 islarge, or when it is difficult to provide a large heating device 30, orwhen the temperature of the work environment is low, it may take time toheat the intermediate green compact 110. In such a case, it is desirableto pre-heat the first die (lower die 21). In the first embodiment, thefirst die is pre-heated for the above reason.

Specifically, a first pre-heating device (not illustrated in thedrawings) that can be adjusted in heating temperature is provided to thefirst die (lower die 21 (cavity 22)), and the first die (lower die 21)is pre-heated after the intermediate green compact 110 has beenobtained, but before the intermediate green compact 110 is transferredto the heating device 30 to pre-heat the lubricant powder. This makes itpossible to reduce the heating time, and reduce the production cycle.

The relationship between the pressure (second pressure P2) applied bythe second press molding device 40 and the density ρ of the resultingfinal green compact 120 is described below with reference to FIG. 4.

The density ρ achieved by the second press molding device 40 has thecharacteristics B. Specifically, the density ρ does not graduallyincrease as the second pressure P2 increases, differing from the case ofusing the first press molding device 20 (see the characteristics A).More specifically, the density ρ does not increase until the final firstpressure P1 (e.g., horizontal axis index: 75 or 85) during the firstpress molding step is exceeded. The density ρ increases rapidly when thesecond pressure P2 has exceeded the final first pressure P1. This meansthat the second press molding step is performed continuously with thefirst press molding step.

Therefore, the first press molding step need not be performed in a statein which the first pressure P1 is necessarily increased to a value(horizontal axis index: 100) corresponding to the maximum capacity. Thismakes it possible to prevent unnecessary time and energy consumptionthat may occur when the first press molding step is continued after thecompression limit has been reached. Therefore, the production cost canbe reduced. Moreover, since it is possible to avoid overloaded operationin which the horizontal axis index exceeds 100, breakage of the die doesnot occur. This makes it possible to ensure easy and stable operation.

A workpiece transfer device 50 can transfer the intermediate greencompact 110 removed from the first die (lower die 21) using the firstejection device (movable member 23 and through-hole 24) to a givenposition within the heating device 30, can transfer the heatedintermediate green compact 110 from the heating device 30 to the seconddie 41 (lower die 41), and can transfer the final green compact 120removed from the second die 41 using the second ejection device (movablemember 43 and through-hole 44) to a discharge section (e.g., dischargechute 59) that discharges the final green compact 120 to the outside ofthe high-density molding system 1. The workpiece transfer device 50 canreliably transfer the workpiece in the area between the first pressmolding device 20 and the heating device 30, the area between theheating device 30 and the second press molding device 40, and the areabetween the second press molding device 40 and the discharge chute 59.

In the first embodiment, the workpiece transfer device 50 is formed bythree transfer bars 51, 52, and 53 (see FIG. 3B) that are operated insynchronization. The transfer bars 51, 52, and 53 are moved to the fronttransfer line (FIG. 3B) from the deep side in FIG. 3A when a transferrequest has been issued, moved from left to right, and then returned tothe original position. A placement device (transfer bar 52, movablemember 43, and through-hole 44) places the heated mixed powderintermediate compressed body (intermediate green compact 110) in thesecond die (lower die 41 (cavity 42)) that is pre-heated to the meltingpoint of the lubricant powder.

Note that the workpiece transfer device may be implemented by a transferdevice that includes a finger that is driven in two-dimensional orthree-dimensional directions, and the like, and sequentially transfersthe workpiece to each die or the like.

It is necessary to take account of unique compression characteristicswhen employing a technique that liquefies the lubricant in theintermediate step. Specifically, when the first pressure P1 is appliedto the intermediate green compact 110 in the presence of the particulatelubricant, the intermediate green compact 110 is compressed rapidly, andthen compressed gradually (see the characteristics A illustrated in FIG.4). When the second pressure P2 is applied to the final green compact120 in the presence of the liquid lubricant, the final green compact 120is compressed rapidly when the first pressure P1 is exceeded, and thenbecomes constant (see the characteristics B illustrated in FIG. 4).

It is technically important to take account of the following facts. Inorder to reduce the overall production cycle consisting of theintermediate green compact-forming step (PR2) and the final greencompact-forming step (PR5), the true density ratio may be increased to acertain degree during the intermediate green compact-forming step (PR2)having the characteristics A, and the final green compact-forming step(PR5) having the characteristics B may then be performed. In this case,if the molding time in which compression occurs slowly increases, it maybe considered to be waste of time. Moreover, breakage of the die mayoccur.

On the other hand, the intermediate green compact-forming step (PR2) maybe stopped rapidly without achieving a high true density ratio, and thefinal green compact-forming step (PR5) may immediately be performedtaking account of the characteristics A and the characteristics B. Inthis case, it is considered that waste of time can be minimized

However, it is considered that a person having ordinary skill in the artdo not sufficiently take account of the characteristics A and thecharacteristics B. Moreover, the operator may arbitrarily select thetiming at which the characteristics B are used instead of thecharacteristics A during actual production. For example, the operatormay change the characteristics from the characteristics A to thecharacteristics B at an early timing in order to immediately stop theintermediate green compact-forming step (PR2). When the intermediategreen compact-forming step (PR2) is stopped at a timing at which thedegree of compression is low, and the final green compact-forming step(PR5) is immediately performed, the following problem occurs during thefinal green compact-forming step (PR5).

Specifically, when the compression ratio (density ratio) of theintermediate green compact 110 is low, it is necessary to relativelyincrease the compression ratio during the final green compact-formingstep (PR5). In this case, the downward stroke of the slide required formolding the final green compact increases. As a result, the relativemoving amount (pressure contact sliding distance) of the outercircumferential surface of the intermediate green compact 110 and theinner wall of the cavity 42 of the second die in the vertical directionincreases by the increase in slide stroke. The friction resistancebetween the outer circumferential surface of the intermediate greencompact 110 and the inner wall of the cavity 42 of the second dierapidly increases in proportion to the increase in the relative movingamount. Specifically, an overloaded state and breakage of the die mayoccur.

Practical application experiments and research were conducted whiletaking account of various parameters such as the type of basic metalpowder, the type and the amount of lubricant, the clearance between thesecond die (lower die 41) and the intermediate green compact 110 in thediametrical direction. It was found that the above problem does notoccur when the first pressure P1 is applied to the mixed powder in thefirst die (lower die 21) so that the density ratio is 85% or more,provided that the maximum density of the mixed powder intermediatecompressed body (intermediate green compact 110) that can be molded byapplying the first pressure P1 is 100%.

In order to prevent occurrence of an overloaded state and breakage ofthe die during the second press molding process, the first press moldingprocess should not be stopped in a state in which the density ratio isless than 85%. The above problem does not occur as long as the densityratio is 85% or more. Note that it is preferable that the density ratiobe 96% or less from the viewpoint of preventing breakage of the die.Since a density ratio of 96% is not a limiting value, but is a boundaryvalue, the upper limit of the density ratio may be selected to be lessthan 100%. The intermediate green compact 110 should not be subjected tothe second press molding process in a state in which the density ratiois less than 85%.

The mixed powder high-density molding system according to the firstembodiment implements the high-density molding method as describedbelow.

<Preparation of Mixed Powder>

The basic metal powder (magnetic-core vitreous insulating film-coatediron powder) and the lubricant powder (zinc stearate powder) (0.2 wt %)are mixed to prepare the mixed powder 100 in a dry state. A given amountof the mixed powder 100 is fed to the mixed powder feeding device 10(step PR0 in FIG. 1).

<Filling with Mixed Powder>

The mixed powder feeding device 10 is moved from a given position(indicated by the solid line in FIG. 3B) to a supply position (indicatedby the broken line in FIG. 3B) at a given timing. The inlet of the mixedpowder feeding device 10 is opened, and the empty lower die 21 (cavity22) of the first press molding device 20 is filled with the mixed powder100 (step PR1 in FIG. 1). The lower die 21 (cavity 22) can be filledwith the mixed powder 100 within 2 seconds, for example. The inlet isclosed after the lower die 21 (22) has been filled with the mixed powder100, and the mixed powder feeding device 10 is returned to the givenposition (indicated by the solid line in FIG. 3B).

<Molding of Intermediate Green Compact>

The upper die 25 of the first press molding device 20 is moved downwardusing the slide 5 illustrated in FIG. 2, and applies the first pressureP1 to the mixed powder 100 in the lower die 21 (cavity 22) (first pressmolding process). The solid lubricant produces a sufficient lubricatingeffect. The density ρ of the compressed intermediate green compact 110increases along the characteristics A illustrated in FIG. 4. In thefirst embodiment, the first press molding process is stopped when thefirst pressure P1 has reached a pressure (3.0 tons/cm²) corresponding toa horizontal axis index of 30, for example. The horizontal axis index 30corresponds to a true density ratio of 85%, and the density ρ hasincreased to 6.63 g/cm³ (vertical axis index: 87). The press moldingprocess is performed for 8 seconds, for example, to obtain theintermediate green compact 110 that has been molded in the die (lowerdie 21) (see FIG. 3A) (step PR2 in FIG. 1). The upper die 25 is thenmoved upward using the slide 5. Note that the second press moldingprocess on the preceding intermediate green compact 110 is performed bythe second press molding device 40 in synchronization with the aboveoperation.

<Removal of Intermediate Green Compact>

The first ejection device (movable member 23) moves the intermediategreen compact 110 upward to the transfer level HL. Specifically, theintermediate green compact 110 is removed from the lower die 21. Theworkpiece transfer device 50 transfers the intermediate green compact110 to the heating device 30 using the transfer bar 51 (see FIG. 3B),and the movable member 23 is returned to the initial position. Theintermediate green compact 110 that has been transferred to the heatingdevice 30 is positioned on the wire-mesh holding member 32 (see FIG.3A).

<Heating>

The heating device 30 starts to operate (see FIG. 3A). Hot air is blownagainst the intermediate green compact 110 from the blow hood 31, sothat the intermediate green compact 110 is heated to the melting point(e.g., 120° C.) of the lubricant powder (step PR3 in FIG. 1).Specifically, the lubricant is melted, and the distribution of thelubricant in the intermediate green compact 110 becomes uniform. Theheating time is 8 to 10 seconds, for example. Note that the hot air isrecycled through the wire-mesh holding member 32 and theexhaust/circulation hood 33.

<Placement of Heated Intermediate Green Compact>

The heated intermediate green compact 110 is transferred to the secondpress molding device 40 by the workpiece transfer device 50 (transferbar 52) (see FIG. 3B), positioned over the lower die 41, and placed onthe movable member 43 in the lower die 41 (cavity 42) (step PR4 in FIG.1).

<Pre-Heating of Die>

The second pre-heating device 47 operates in the second press moldingdevice 40 (optional). The second pre-heating device 47 heats the die(lower die 41 (cavity 42)) to the melting point (120° C.) of thelubricant powder before the intermediate green compact 110 is placed inthe die (lower die 41 (cavity 42)). This makes it possible to preventsolidification of the lubricant included in the heated intermediategreen compact 110 placed in the second die (lower die 41 (cavity 42)).

<Molding of Final Green Compact>

The upper die 45 is moved downward using the slide 5 illustrated in FIG.2 (see FIG. 3A), and applies the second pressure P2 to the intermediategreen compact 110 in the lower die 41 (cavity 42). The liquid lubricantproduces a sufficient lubricating effect. The density ρ of thecompressed intermediate green compact 110 increases along thecharacteristics B illustrated in FIG. 4. Specifically, when the secondpressure P2 has exceeded a horizontal axis index of 30 (3.0 tons/cm²),for example, the density ρ corresponding to a true density ratio of 85%rapidly increases from 6.63 g/cm³ to a value (7.75 g/cm³) correspondingto a vertical axis index of 102. When the second pressure P2 isincreased to a horizontal axis index of 100 (10 tons/cm²), the density ρ(7.75 g/cm³) becomes uniform over the entire green compact. Since therequired slide stroke (relative movement amount) is short, an overloadedstate and breakage of the die do not occur. Since the sweatingphenomenon (i.e., the lubricant flows out in all directions) occursduring the press molding process, the friction resistance between thebasic metal particles and the die and the friction resistance betweenthe particles and the die can be efficiently reduced. The second pressmolding process is performed for 8 seconds, for example, to obtain thefinal green compact 120 that has been molded in the second die (lowerdie 41) (step PR5 in FIG. 1). The upper die 45 is then moved upwardusing the slide 5. Note that the first press molding process on thesubsequent intermediate green compact 110 is performed by the firstpress molding device 20 in synchronization with the above operation.

<Removal of Product>

The second ejection device (movable member 43) moves the final greencompact 120 upward to the transfer level HL. Specifically, the finalgreen compact 120 is removed from the lower die 41. The workpiecetransfer device 50 transfers the final green compact 120 to thedischarge chute 59 using the transfer bar 53 (see FIG. 3B), and themovable member 43 is returned to the initial position. The vitreousmaterial included in the final green compact 120 having a density ρ of7.75 g/cm³ corresponding to a vertical axis index of 102 is notmodified/melted since the melting point of the lubricant powder was low.Therefore, a high-quality magnetic-core green compact that can reduceeddy current loss and improve magnetic flux density can be efficientlyproduced.

<Production Cycle>

According to the high-density molding method, since the first pressmolding process, the heating process, and the second press moldingprocess can be performed in synchronization on the metal powder 100(mixed powder 100) that is sequentially fed, the high-density greencompact (final green compact 120) can be produced in a cycle time of 12to 14 seconds (i.e., maximum heating time (e.g., 10 seconds)+workpiecetransfer time (e.g., 2 to 4 seconds)). This makes it possible toremarkably reduce the production time as compared with the related-artexample (high-temperature sintering time: 30 minutes or more). Forexample, it is possible to ensure a stable supply of automotive partsthat have a reduced size and weight, a complex shape, and highmechanical strength, or electromagnetic device parts that exhibitexcellent magnetic properties and mechanical strength, and significantlyreduce the production cost.

The high-density molding method according to the first embodiment canreliably and stably produce a high-density green compact whilesignificantly reducing the production cost by filling the first die(lower die 21) with the mixed powder 100 prepared by mixing thelow-melting-point lubricant powder into the basic metal powder, applyingthe first pressure P1 to the mixed powder 100 in the first die (lowerdie 21) to form the intermediate green compact 110 having a densityratio of 85 to 96%, provided that the maximum density of theintermediate green compact that can be molded by applying the firstpressure is 100%, heating the intermediate green compact 110 to themelting point (e.g., 120° C.) of the lubricant powder, placing theheated intermediate green compact 110 in the second die (lower die 41),and applying the second pressure P2 to the intermediate green compact110 in the second die (lower die 41) to form the final green compact120. It is also possible to reduce the actual production cycle whileensuring the safety of the device (e.g., die).

Since a sintering process that is performed at a high temperature for along time can be made unnecessary, oxidation of the green compacts 110and 120 can be significantly suppressed while minimizing energyconsumption, and significantly reducing the production cost. This isadvantageous from the viewpoint of environmental protection.

Since the lubricant powder has a low melting point within the range of90 to 190° C., it is possible to suppress oxidation of the lubricantwhile enhancing the selectivity of the lubricant.

Since the second die (lower die 41) is pre-heated using the secondpre-heating device 47, it is possible to further improve the fluidity ofthe lubricant in all directions during the second press molding step.This makes it possible to significantly reduce the friction resistancebetween the basic metal particles and the friction resistance betweenthe particles and the second die.

Since the second pressure P2 is set to be equal to the first pressureP1, it is possible to easily implement the press molding step,facilitate handling, indirectly reduce the green compact productioncost, and easily implement the system based on a single press, forexample.

The high-density molding method according to the first embodiment canefficiently and stably produce a magnetic core part that exhibitsexcellent magnetic properties corresponding to the type of basic metalpowder, using a magnetic-core vitreous insulating film-coated ironpowder, a magnetic-core iron-based amorphous powder, or a magnetic-coreFe—Si alloy powder as the basic metal powder.

It has been impossible to achieve a density equal to or higher than thatcorresponding to a vertical axis index of 100, taking account of thecapacity (horizontal axis index=100 (see FIG. 4)) of a related-artsystem (e.g., press). According to the first embodiment, however, it ispossible to achieve a density equal to or higher than that correspondingto a vertical axis index of 102 using an identical (existing) system.This fact achieves a major breakthrough in the technical field.

The high-density molding system 1 that includes the mixed powder feedingdevice 10, the first press molding device 20, the heating device 30, andthe second press molding device 40 can reliably and stably implement thehigh-density molding method.

Second Embodiment

FIG. 6 illustrates a second embodiment of the invention. The secondembodiment is identical with the first embodiment as to the mixed powderfeeding device 10 and the first press molding device 20, but differsfrom the first embodiment in that the heating device 30 and the secondpress molding device 40 are integrally formed.

Specifically, a high-density molding system according to the secondembodiment includes a heating/press molding device 70 that has thefunction of the heating device 30 and the function of the second pressmolding device 40 (see the first embodiment). The heating/press moldingdevice 70 includes a plurality of (e.g., two) heating/press moldingsub-devices 70A and 70B. The heating/press molding sub-devices 70A and70B are selectively (sequentially) operated by a control device (notillustrated in the drawings) in a production cycle.

The heating/press molding sub-devices 70A and 70B have a basic structuresimilar to that of the second press molding device 40 described above inconnection with the first embodiment. Each of the heating/press moldingsub-devices 70A and 70B includes a hybrid heating device 48 having thefunctions of the heating device 30 and the second pre-heating device 47described above in connection with the first embodiment.

Specifically, the hybrid heating device 48 is an electric heating devicehaving a heating temperature switch function. The hybrid heating device48 can pre-heat the lower die 41 to the melting point (e.g., 120° C.) ofthe lubricant in advance (i.e., before the intermediate green compact110 is placed in the lower die 41). When the intermediate green compact110 has been placed in the lower die 41, the amount of heat is changedso that the entire intermediate green compact 110 can be heated to themelting point (e.g., 120° C.) of the lubricant. The heating target areacan also be selected (changed). After completion of the heating process,the second press molding process is performed using the second pressmolding device 40 in the same manner as described above in connectionwith the first embodiment. The hybrid heating device 48 can maintain theintermediate green compact 110 at a temperature equal to or higher thanthe melting point (e.g., 120° C.) of the lubricant during the secondpress molding process.

As illustrated in FIG. 6, each heating/press molding sub-device (20,70A, 70B) has an independent press structure, and each slide (5, 5A, 5B)is independently moved upward and downward by controlling the rotationof each motor. Specifically, when one of the heating/press moldingsub-devices 70A and 70B performs the press molding operation, the otherof the heating/press molding sub-devices 70A and 70B performs thepre-heating operation, and does not perform the press molding operation.This also applies to the case where the heating/press molding device 70is implemented by three or more heating/press molding sub-devices takingaccount of the production cycle time.

In the second embodiment, when the third intermediate green compact 110is press-molded in the first press molding device 20, the secondintermediate green compact 110 is heated by the heating/press moldingsub-device 70A (or the heating/press molding sub-device 70B), and thefirst intermediate green compact 110 is press-molded by theheating/press molding sub-device 70B (or the heating/press moldingsub-device 70A) to form the final green compact 120.

According to the second embodiment, since the heating/press moldingdevice 70 is implemented by a plurality of heating/press moldingsub-devices 70A and 70B having an identical structure, the system can besimplified as compared with the first embodiment. It is also possible tosimplify the production line, and further facilitate handling.

Note that the first press molding device 20 and the heating/pressmolding sub-device 70A (or the heating/press molding sub-device 70B), orthe first press molding device 20 and the heating/press moldingsub-devices 70A and 70B may be implemented by a single press structure.

REFERENCE SIGNS LIST

-   1 High-density molding system-   10 Mixed powder feeding device-   20 First press molding device-   30 Heating device-   40 Second press molding device-   47 Pre-heating device-   48 Hybrid heating device-   50 Workpiece transfer device-   70 Heating/press molding device-   70A, 70B Heating/press molding sub-device-   100 Mixed powder-   110 Intermediate green compact (mixed powder intermediate compressed    body)-   120 Final green compact (mixed powder final compressed body)

1. A mixed powder high-density molding method comprising: filling afirst die with a mixed powder prepared by mixing a low-melting-pointlubricant powder into a basic metal powder; applying a first pressure tothe mixed powder in the first die to form a mixed powder intermediatecompressed body having a density ratio of 85 to 96%, provided that amaximum density of the mixed powder intermediate compressed body thatcan be molded by applying the first pressure is 100%; heating the mixedpowder intermediate compressed body removed from the first die to amelting point of the lubricant powder; placing the heated mixed powderintermediate compressed body in a second die; and applying a secondpressure to the mixed powder intermediate compressed body in the seconddie to form a high-density mixed powder final compressed body.
 2. Themixed powder high-density molding method as defined in claim 1, whereinthe lubricant powder has a low melting point within a range of 90 to190° C.
 3. The mixed powder high-density molding method as defined inclaim 1, wherein the second die is pre-heated to the melting pointbefore the mixed powder intermediate compressed body is placed in thesecond die.
 4. The mixed powder high-density molding method as definedin claim 1, wherein the second pressure is selected to be equal to thefirst pressure.
 5. A mixed powder high-density molding systemcomprising: a mixed powder feeding device that can externally feed amixed powder prepared by mixing a low-melting-point lubricant powderinto a basic metal powder; a first press molding device that applies afirst pressure to the mixed powder, with which a first die has beenfilled using the mixed powder feeding device, to form a mixed powderintermediate compressed body; a heating device that heats the mixedpowder intermediate compressed body removed from the first die to amelting point of the lubricant powder; and a second press molding devicethat applies a second pressure to the mixed powder intermediatecompressed body that is placed in a second die to form a high-densitymixed powder final compressed body, the first press molding deviceapplying the first pressure to the mixed powder in the first die to formthe mixed powder intermediate compressed body having a density ratio of85 to 96%, provided that a maximum density of the mixed powderintermediate compressed body that can be molded by applying the firstpressure is 100%.
 6. The mixed powder high-density molding system asdefined in claim 5, wherein the heating device and the second pressmolding device are formed by a heating/press molding device thatfunctions as the heating device and the second press molding device, theheating/press molding device includes a plurality of heating/pressmolding sub-devices, and each of the plurality of heating/press moldingsub-devices can be selectively and sequentially operated in each cycle.7. The mixed powder high-density molding system as defined in claim 5,further comprising: a pre-heating device that pre-heats the second die.8. The mixed powder high-density molding system as defined in claim 5,further comprising: a workpiece transfer device that transfers the mixedpowder intermediate compressed body formed by the first press moldingdevice to the heating device, transfers the mixed powder intermediatecompressed body heated by the heating device to the second press moldingdevice, and transfers the mixed powder final compressed body formed bythe second press molding device to a discharge section.